In some tank applications, access to the tank is provided through a relatively long (e.g. 2-3 m) and narrow (e.g. 5-10 cm) nozzle. One such application is underground storage tanks, which are used for storing liquid products in a very cold environment.
In order to monitor the filling level in such tanks, a conventional float or level switch is typically used. While providing some information about the filling level, such as when a critical level is exceeded, such level switches cannot provide reliable information about the filling level.
Attempts have been made to employ a guided wave radar (GWR) level gauge to provide filling level information. A GWR level gauge includes a transmission line (probe) extending into the tank, for guiding electromagnetic transmit signals and returning a reflection from an interface between tank atmosphere and product in the tank. However, is difficult to obtain reliable measurements due to interference caused by the long nozzle. Also, temperature gradients along the nozzle, typically extending from an underground level up to ground level, may cause condensation along the probe causing further interference.
U.S. Pat. No. 6,690,320 discloses a system attempting to solve some of these issues by confining the probe within a coaxial extension down to a position below the nozzle. Hereby, the electromagnetic field propagating along the probe is not affected by the nozzle, and the result is the same as if the probe had been installed below the nozzle.
However, the solution in U.S. Pat. No. 6,690,320 does not overcome the problem of temperature gradients causing condensation and possibly ice along the probe in the nozzle.
Also, in some situations the difficulty of containing and cleaning any underground leakage makes it extremely important to prevent any overfill condition. Any gauging system provided in such a tank must therefore be sufficiently robust.